HARINAGAR, BANGLADESH—One by one, the men in Gaurpodomando's family walked out of this mud-caked village and never returned.

First, his uncles went. Both fishermen, they suffered as their catch declined year after year, before they crossed illegally into India to find work in construction. His brothers earned so little fishing that they braved tiger attacks in the nearby Sundarbans forest to forage for honey and timber. Finally, they left, too, and brought their father with them.

Now, Gaurpodomando, who said he is about 35 years old and who goes only by his first name, is the last man in his family still living in the waterlogged village along Bangladesh's Indian border.

His brothers still don't know about the angry tidal flood that burst through a dam and swallowed the family home and dozens of others in September. Those who live here say that between the disappearing fish, brackish floodwaters destroying the rice fields and the ever-fiercer cyclones that seem to inhale entire villages, life is becoming almost unbearable.

But Gaurpodomando, who earns the equivalent of $1.50 a day standing hip-deep in the salty river casting a net to collect shrimp fry, said he is doing everything he can to hang onto his way of life.

"I do feel a little lonely and sad, but I don't really want to go to India," he said, squatting on the outdoor stoop of what was once the family kitchen but is now the only structure left to shelter him, his wife and their two children. His arms and bare feet are streaked with the slate-gray mud that covers the ground and seems never to dry.

'I don't want to leave this country'

"I don't want to leave this place," Gaurpodomando said. "I don't want to leave this country. I love this place."

One day soon, Gaurpodomando and an untold number of others in Bangladesh and around the world may no longer have a choice.

Some of the big questions remain unanswered: How many people will really move? Where will they go? How will they go? Will they return?

But experts estimate that as many as 250 million people—a population almost that of the entire United States—could be on the move by 2050. They will go because temperatures are rising and desertification has set in where rainfall is needed most. They will go because more potent monsoons are making flood-prone areas worse. They will go because of other water events caused by melting glaciers, rising seas and the slow and deadly seepage of saline water into their wells and fields.

The worst migration cases will be nations like the Maldives and small islands in the Pacific. Their inhabitants will go because their homelands will likely sink beneath the rising sea.

According to the Intergovernmental Panel on Climate Change, a minimum of 207 million people in Latin America, Asia and Africa will not have enough water inside a decade. In Asia, an extra 130 million people will be at risk of hunger by the middle of the century. By 2100, crop revenues in Africa will drop 90 percent. And scientists see Bangladesh as ground zero.

The country's 150 million inhabitants live in the delta of three waterways about the size of Iowa, and the majority of the country sits less than 20 feet above sea level. According to the IPCC, rising sea levels will wipe out more cultivated land in Bangladesh than anywhere in the world. By 2050, rice production is expected to drop 10 percent and wheat production by 30 percent.

By the end of the century, more than a quarter of the country will be inundated.

About 15 million people in Bangladesh alone could be displaced. That's the equivalent of every person in New York, Los Angeles and Chicago.

A migration that will change the face of the world

But while more climate migrants will come from Bangladesh than from any other country, scientists say that from Mozambique to Tuvalu, from Egypt to Vietnam, climate migration will change the face of the world.

"This will be the largest migration in history. This is not migration as we've known it before," said Edward Cameron, a former senior adviser to the government of the Maldives. "We're talking about people migrating from sensitive places into other very sensitive places."

In some ways, large-scale migration is nothing new. Humans, after all, have been on the move since early man left East Africa. But these shifts will not be the migrations of pioneers or adventurers seeking opportunities in new lands. Rather, social scientists say, they will be the movement of people who are rushed, unwanted and unprepared, into unfamiliar and perhaps hostile new environments. Most of those who will be uprooted already are living on less than $1 per day.

The first shifts will start within countries. Scientists see families flocking from rural and coastal areas to cities where livelihoods are less tied to fickle weather patterns. It's a pattern that is already happening against a background of rapid global urbanization, in which the desperate rate of growth far outpaces jobs and infrastructure.

Mohammad Ayub Ali, 40, is part of that mosaic. He left the central Bangladesh town of Sherpur because the failing crops couldn't earn him a living. A ruinous flood in September was the final straw.

Now Ali drives an eye-catching pink and orange rickshaw through the capital city Dhaka's teeming streets, where he earns the equivalent of $15 per month. He lives in a one-room metal shack with his mother, wife and two children.

"It's not that great over here, but it's better than over there," he said. Nearly 3.5 million people in Dhaka—about 40 percent of the population—live in slums, like Ali. The World Bank estimates that by midcentury, half of all Bengalis will live in urban centers.

Moving from one area of resource scarcity to another

The next step in the migration pattern is across national borders. Military experts predict a downward spiral of violence and conflict as people desperate for food, water and jobs cross into neighboring countries where resources may be only slightly less scarce.

Wealthy nations like the United States and the European Union, meanwhile, could also be asked to take in millions of the world's displaced people even as they negotiate international disputes.

"Those people who are most vulnerable right now, and having a problem just surviving, and having the normal development challenges of clean water, fighting disease, getting an education—those are the ones most affected," said Koko Warner, who heads the Environmental Migration, Social Vulnerability and Adaptation Section at the U.N. University.

In Bangladesh, the issues are magnified by the density of the population. Any climate-induced disaster "inevitably affects millions of people," researcher James Pender wrote in a recent sweeping report on Bangladesh. He estimated that by 2080, almost all the 51 million to 97 million people currently living in coastal zones may have to leave. The worst off won't even be able to do that.

"If those who are causing the greenhouse gas emissions are unable to control carbon emissions, the people in the vulnerable areas, many of the coastal areas, are going to be inundated," said Khawaja Minnatullah, a water specialist at the World Bank's Dhaka office.

"The vulnerable, the uneducated, the lowest of the communities will never be able to migrate to the U.S., to Canada, to Australia. There will be pressure on the not-so-vulnerable part of Bangladesh," he said.

In the village of Gabura in southwest Bangladesh, 20-year-old Amina lives with the fractured collarbone she suffered when a tidal flood smashed a wall of her home, crushing her. She and her husband have no money for a doctor, much less a move.

"Everyone that's living here, we're all poor people," she said, sitting in front of her partially repaired mud and thatch house. "We don't have anywhere to go."

Swelling overcrowded cities; scaring neighbors who have built a fence

But in Gabura and other parts of Bangladesh where the land can become the sea in the blink of an eye, climate migration has already begun.

Cities like Dhaka are bursting at the seams. Migration to bordering India appears to be occurring at a higher rate, as well, though government leaders are reluctant to acknowledge it. India, meanwhile, is wide awake to the possibility of migration from Bangladesh, and is building a fence much like the one along the U.S.-Mexico border to keep illegal immigrants out.

There is a human tendency to deny mind-numbing futures like this one, and Bangladeshi experts are positioned on both sides of this verbal fence. Some insist that climate migration is a reality that needs to be addressed sooner than later.

Others say a large-scale migration out of the country will mean the world has failed to tackle global warming. It's a prospect they don't even want to acknowledge. "This idea of climate refugees take up too much of our time. It's an apocalyptic issue of the future," said Omar Rahman, dean of the Independent University, Bangladesh, in Dhaka.

Ainun Nishat, the International Union for Conservation of Nature's representative for Bangladesh, said he is skeptical of migration predictions. Even if they are true, he argued, Bangladesh's needs are more immediate: infrastructure improvements, cyclone shelters, improved flood warning systems and a massive build-up of food security.
'It's not time to worry about it'

"Will people leave? Maybe in a hundred years, but that's not my priority now," Nishat said. "People are living in areas that go underwater once in a fortnight in the coastal belt. The point is, they're still there. They're not migrating today.

"It's not time to worry about it. My priority is the natural disaster that is happening now."

This year, the Western world will continue to grapple with the issue. U.S. President Barack Obama will try to convince Congress to pass a domestic cap-and-trade bill. Meanwhile, the European Union is struggling to implement a plan on reducing emissions. In China and India, which have the economic muscle to begin some actions, debates continue to rage over how much responsibility to bear for fast-rising emissions.

There is little news about this here in Harinagar, where men and women said they probably won't be able to wait for politicians to agree on a global solution. Like the proverbial grains of sand that slowly assemble to make up a beach, individual families are making their painful decisions, creating the possibility of more cruel things yet to come.

"The area is getting worse. I don't think it's going to get better," Gaurpodomando said. His wife, Chorna, her face loosely framed by a red floral headscarf, bounced the couple's 3-year-old daughter on her hip and said she, too, wants to stay, but she's also realistic about the family's prospects. Maybe, she said, they'll go to Khulna, a booming port city about two hours away by car.

Gaurpodomando said his brothers living outside of Kolkata "say it's good over there. They keep asking me to go, and they tell me there's good earning to be done there."

But Harinagar, where the thatched mud huts still look out over a lush countryside, and where a woman who lost everything in a recent flood will still offer a visiting stranger a plate of eggs, has been his family's home for at least three generations.

"I'll do whatever work I can find, but I might have to go outside," Gaurpodomando said. "We might have to leave this village."

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